Visibly transparent, near-infrared-absorbing metal-complex photovoltaic devices

ABSTRACT

Visibly transparent photovoltaic devices are disclosed, such as those are transparent to visible light but absorb near-infrared light and/or ultraviolet light. The photovoltaic devices make use of transparent electrodes and near-infrared absorbing visibly transparent photoactive compounds, optical materials, and/or buffer materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication No. 62/521,158, filed on Jun. 16, 2017, which is herebyincorporated by reference in its entirety.

FIELD

This application relates generally to the field of optically activematerials and devices, and, more particularly, to visibly transparentphotovoltaic devices and materials for visibly transparent photovoltaicdevices.

BACKGROUND

The surface area necessary to take advantage of solar energy remains anobstacle to offsetting a significant portion of non-renewable energyconsumption. For this reason, low-cost, transparent, organicphotovoltaic (OPV) devices that can be integrated onto window panes inhomes, skyscrapers, and automobiles are desirable. For example, windowglass utilized in automobiles and architecture are typically 70-80% and55-90% transmissive, respectively, to the visible spectrum, e.g., lightwith wavelengths from about 450 to 650 nanometers (nm). The limitedmechanical flexibility, high module cost and, more importantly, theband-like absorption of inorganic semiconductors limit their potentialutility to transparent solar cells.

In contrast, the optical characteristics of organic and molecularsemiconductors results in absorption spectra that are highly structuredwith absorption minima and maxima that are uniquely distinct from theband absorption of their inorganic counterparts. However, a variety oforganic and molecular semiconductors exist, but many of exhibit strongabsorption in the visible spectrum and thus are not optimal for use inwindow glass-based photovoltaics.

SUMMARY

Described herein are materials, methods, and systems related to visiblytransparent photovoltaic devices. More particularly, the presentdescription provides metal dithiolate compounds and methods and systemsincorporating the materials and compounds as a photoactive material of avisibly transparent photovoltaic device. Embodiments of the presentinvention provide transparent photoactive materials, including organicsemiconductor materials, useful in visibly transparent photovoltaicdevices. The invention is applicable to a variety of applications inphotovoltaics and electronics.

Materials, methods, and system for transparent photovoltaic devices aredisclosed, such as those that are transparent to visible light butabsorb near-infrared light and/or ultraviolet light. The photovoltaicdevices make use of transparent electrodes and visibly transparent metaldithiolate compounds, which may be useful as photoactive materials,buffer materials, and/or optical layers, for example. In embodiments,the disclosed visibly transparent photoactive compounds exhibit amaximum near-infrared absorption strength and a maximum visibleabsorption strength, wherein the maximum near-infrared absorptionstrength is greater than the maximum visible absorption strength.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentinvention provide methods and systems for absorbing near-infrared and/orultraviolet radiation for photovoltaic power generation while beingtransparent to visible light. Advantageously, these opticalcharacteristics provide for the ability to generate electricity fromincident solar radiation in a photovoltaic device, while still allowinguseful visible light to be passed through, permitting a viewer to seethrough the photovoltaic device.

In addition, the photoactive compounds described herein provide suitableelectron donor and/or electron acceptors for generation and separationof electron-hole pairs via absorption of light in order to provide a DCvoltage or current to an external circuit. Advantageously, the disclosedphotoactive compounds include those that are transparent to visiblelight or absorb only relatively small amounts of light in the visibleband, such as between about 450 and about 650 nm, while exhibiting agreater absorption strength in the near-infrared (NIR) band, such asbetween about 650 nm and about 1400 nm. In some embodiments, thedisclosed photoactive compounds exhibit a peak absorption in the NIRband. In some embodiments, the disclosed photoactive compounds exhibitan absorption in the visible band having a strength less than that inthe NIR band.

The disclosed photoactive compounds exhibit materials propertiesproviding advantages for manufacturing and performance of visiblytransparent photovoltaic devices as well. For example, devices includingphotoactive compounds described herein may be manufactured usingtechniques in which the photoactive compound is applied to a substrateusing vacuum deposition techniques rather than a solution-basedapplication technique. Use of vacuum deposition techniques mayadvantageously allow high purity photoactive layers to be generated,improving device efficiency and performance and reducing manufacturingcomplexity. For example, transparent photovoltaic devices mayincorporate the disclosed photoactive compounds into active materiallayers by way of vacuum thermal evaporation techniques, eliminating useof solution based photoactive material application steps and associatedwaste product handling and disposal. Further, in some embodiments, thedisclosed photoactive compounds may be purified by an evaporation and/orsublimation technique. Purification by evaporation and/or sublimationmay be useful for generating high purity photoactive materials andcompounds, which may, in turn, allow improved transparent photovoltaicdevice production and performance.

A visibly transparent photovoltaic device may comprise a visiblytransparent substrate; a first visibly transparent electrode coupled tothe visibly transparent substrate; a second visibly transparentelectrode above the first visibly transparent electrode; a first visiblytransparent active layer between the first visibly transparent electrodeand the second visibly transparent electrode, wherein the first visiblytransparent photoactive layer comprises a visibly transparentphotoactive compound, such as any visibly transparent photoactivecompound described herein, including a visibly transparent photoactivecompound that exhibits a first maximum near-infrared absorption strengthand a first maximum visible absorption strength, such as a first maximumnear-infrared absorption strength that is greater than the first maximumvisible absorption strength; a second visibly transparent photoactivelayer between the first visibly transparent electrode and the secondvisibly transparent electrode and exhibiting a second maximum absorptionstrength in the near-infrared or ultraviolet and a second maximumvisible absorption strength, such as a second maximum absorptionstrength that is greater than the second maximum visible absorptionstrength. Optionally, visibly transparent photovoltaic devices mayfurther comprise a first buffer layer disposed between the first visiblytransparent electrode and the first visibly transparent active layer.Optionally, visibly transparent photovoltaic devices may furthercomprise a second buffer layer disposed between the second visiblytransparent active layer and the second visibly transparent electrode.

Optionally, the first visibly transparent photoactive layer is coupledto the first visibly transparent electrode. Optionally, the firstvisibly transparent photoactive layer is coupled to the second visiblytransparent electrode. Optionally, the second visibly transparentphotoactive layer is coupled to the first visibly transparent electrode.Optionally, the second visibly transparent photoactive layer is coupledto the second visibly transparent electrode. As used herein, two objectsor layers being coupled refers to the condition where the two objects orlayers are directly coupled (i.e., in physical contact) or indirectlycoupled, such as with one or more additional objects or layers betweenthem. Coupling of objects or layers may indicate that the objects orlayers are part of a single overall structure or device.

Optionally, the photoactive compound functions as an electron acceptormaterial. In some embodiments, when the photoactive compound functionsas an electron acceptor material, the second visibly transparentphotoactive layer may include an electron donor material. Optionally,the photoactive compound functions as an electron donor material. Inembodiments, when the photoactive compound functions as an electrondonor material, the second visibly transparent photoactive layer mayinclude an electron acceptor material.

Various configurations are contemplated for the photoactive layers. Forexample, the first visibly transparent photoactive layer and the secondvisibly transparent photoactive layer may each independently havethicknesses of 1 nm to 300 nm, such as about 5 nm, 10 nm, 15 nm, 20 nm,25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 105 nm, 110 nm, 115 nm, 120 nm,125 nm, 130 nm, 135 nm, 140 nm, 145 nm, 150 nm, 155 nm, 160 nm, 165 nm,170 nm, 175 nm, 180 nm, 185 nm, 190 nm, 195 nm, 200 nm, 205 nm, 210 nm,215 nm, 220 nm, 225 nm, 230 nm, 235 nm, 240 nm, 245 nm, 250 nm, 255 nm,260 nm, 265 nm, 270 nm, 275 nm, 280 nm, 285 nm, 290 nm, 295 nm, or 300nm. Optionally, the first visibly transparent photoactive layer and thesecond visibly transparent photoactive layer correspond to separate,mixed, or partially mixed layers.

A variety of compounds are useful as the photoactive compounds in thedescribed visibly transparent photovoltaic devices. For example, thephotoactive compound may optionally exhibit a peak absorption in thenear-infrared. Optionally, the photoactive compound may exhibit a peakabsorption in the ultraviolet.

Example photoactive compounds described herein and useful for aphotoactive layer in a visibly transparent photovoltaic device includethose incorporating metal dithiolate complexes, also referred to asmetal dithiolate compounds or metal dithiolene compounds, such as nickeldithiolate compounds, cobalt dithiolate compounds, iron dithiolatecompounds, zinc dithiolate compounds, molybdenum dithiolate compounds,palladium dithiolate compounds, platinum dithiolate compounds, etc. Insome examples, a photoactive compound provided herein has the formula

In embodiments, each M is independently a metal, such as Fe, Co, Ni, Cu,Zn, Mo, Pd, or Pt. In embodiments, each R group is independently ahydrogen, a halogen, a cyano group, a substituted or unsubstituted alkylgroup, a substituted or unsubstituted alkenyl group, a substituted orunsubstituted aromatic group, a substituted or unsubstituted fusedaromatic group, a substituted or unsubstituted heteroaromatic group, asubstituted or unsubstituted fused heteroaromatic group, or asubstituted or unsubstituted alkoxy group. Optionally, two or more Rgroups form a substituted or unsubstituted ring group or a substitutedor unsubstituted fused ring group. In embodiments, each A isindependently a fused polycyclic ring group having one or more R groupsubstituents. Optionally, one or more R groups are each independently amethoxy group, a dimethyl amino group, a cyano group a thiazolyl group,a phenyl group, a maleimide group, a naphthyl group, or a phenanthrenegroup.

A visibly transparent photoactive compound having the formula:

may be further characterized by the formula:

A visibly transparent photoactive compound having the formula:

may be further characterized by the formula

where each A is independently a fused polycyclic ring group having oneor more R group substituents. For example, the visibly transparentphotoactive compound may have the formula

Specific photoactive compounds include, but are not limited to

In another aspect, visibly transparent photoactive compounds areprovided, which may be useful, for example, as photoactive materials ina photoactive layer of a visibly transparent photovoltaic device. Aphotoactive compound of this aspect may have the formula:

where M is a metal, each A is independently a fused polycyclic ringgroup having one or more R group substituents, and each R group isindependently a hydrogen, a halogen, a cyano group, a substituted orunsubstituted alkyl group, a substituted or unsubstituted alkenyl group,a substituted or unsubstituted aromatic group, a substituted orunsubstituted fused aromatic group, a substituted or unsubstitutedheteroaromatic group, a substituted or unsubstituted fusedheteroaromatic group, or a substituted or unsubstituted alkoxy group.Optionally, two or more R groups form a substituted or unsubstitutedring group or a substituted or unsubstituted fused ring group.Optionally, M is Fe, Co, Ni, Cu, Zn, Mo, Pd, or Pt. Optionally, one ormore R groups are each independently a methoxy group, a phenyl group, adimethyl amino group, a cyano group a thiazolyl group, or a phenylgroup. Optionally, A includes a thiazolyl group, a phenyl group, amaleimide group, a naphthyl group, an acenaphthalene group, or aphenanthrene group. It will be appreciated that these compounds mayexhibit stronger maximum absorption strengths in the near-infrared thanmaximum absorption strengths in the visible.

For example, a photoactive compound of this aspect may have the formula:

Optionally, a compound of this aspect has the formula:

Specific visibly transparent photoactive compounds include those havingthe formula:

Again, it will be appreciated that these compounds may exhibit strongermaximum absorption strengths in the near-infrared than maximumabsorption strengths in the visible.

A photoactive compound of this aspect may alternatively have theformula:

where M is a metal, such as Fe, Co, Ni, Cu, Zn, Mo, Pd, or Pt, each Rgroup is independently a hydrogen, a halogen, a cyano group, asubstituted or unsubstituted alkyl group, a substituted or unsubstitutedalkenyl group, a substituted or unsubstituted aromatic group, asubstituted or unsubstituted fused aromatic group, a substituted orunsubstituted heteroaromatic group, a substituted or unsubstituted fusedheteroaromatic group, or a substituted or unsubstituted alkoxy group, orwherein two or more R groups form a substituted or unsubstituted ringgroup or a substituted or unsubstituted fused ring group. Optionally,one or more R groups are independently a methoxy group, a phenyl group,a dimethyl amino group, a cyano group a thiazolyl group, or a phenylgroup. It will be appreciated that these compounds may exhibit strongermaximum absorption strengths in the near-infrared than maximumabsorption strengths in the visible.

For example, a photoactive compound of this aspect may have the formula:

Optionally, a photoactive compound has the formula

Specific photoactive compounds include those having the formula:

Again, it will be appreciated that these compounds may exhibit strongermaximum absorption strengths in the near-infrared than maximumabsorption strengths in the visible.

Advantageously, the disclosed photoactive compounds exhibit greatermaximum or peak near-infrared absorption strength than the maximum orpeak visible absorption strength. In some embodiments, the disclosedphotoactive compounds exhibit an integrated near-infrared absorptioncoefficient that is great than an integrated visible absorptioncoefficient. For example, some embodiments of the disclosed photoactivecompounds exhibit peak absorption outside a visible wavelength band (450nm to 650 nm), such as in the near-infrared wavelength band (650 nm to1400 nm). By preferentially absorbing near-infrared radiation, thedisclosed photoactive compounds may exhibit visible transparency andadvantageously be useful for incorporation in visibly transparentphotovoltaic devices. It will be appreciated, in various embodiments,that different maximum visible absorption or minimum visibletransmittance may be selected for a particular target application. Forexample, in some embodiments, a visible transmittance of 30% orabsorption of 70% may be desirable for some applications. In otherembodiments, a visible transmittance of 100% or absorption of 0% may bedesirable for some applications. In this way, a visibly transparentphotovoltaic cell may absorb some visible light as well as near-infraredand/or ultraviolet light for purposes of photovoltaic generation (orincidental absorption), and still meet a target visible transparencylevel.

As used herein, the terms visible transparency, visibly transparent, andthe like refer to a character of a material that exhibits an overallabsorption, average absorption, or maximum absorption in the visibleband of 0%-70%, such as less than or about 70%, less than or about 65%,less than or about 60%, less than or about 55%, less than or about 50%,less than or about 45%, less than or about 40%, less than or about 35%,less than or about 30%, less than or about 25%, or less than or about20%. Stated another way, visibly transparent materials may transmit30%-100% of incident visible light, such as greater than or about 80% ofincident visible light, greater than or about 75% of incident visiblelight, greater than or about 70% of incident visible light, greater thanor about 65% of incident visible light, greater than or about 60% ofincident visible light, greater than or about 55% of incident visiblelight, greater than or about 50% of incident visible light, greater thanor about 45% of incident visible light, greater than or about 40% ofincident visible light, greater than or about 35% of incident visiblelight, or greater than or about 30% of incident visible light. Visiblytransparent materials are generally considered at least partiallysee-through (i.e., not completely opaque) when viewed by a human.Optionally, visibly transparent materials may be colorless (i.e., notexhibit strong visible absorption features that would provide anappearance of a particular color) when viewed by a human.

Various different R-group substitutions are contemplated herein. Forexample, in some embodiments, one or more R groups are independently H,CH₃, F, Cl, CN, OCH₃, C(CH₃)₃, or Ar—R, wherein Ar is an aromatic orheteroaromatic group, and wherein Ar—R corresponds to the aromatic orheteroaromatic group having one or more R group substituents.Optionally, one or more R groups are independently a thiazolyl group, aphenyl group, a pyridinyl group, an imidazolyl group, a pyrrolyl group,a thiophenyl group, a naphthyl group, a pyrenyl group, an indolyl group,a benzothiophenyl group, a benzimidazolyl group, or a benzothiazolylgroup. In some embodiments, two or more R groups form an unfusedaromatic or heteroaromatic group or two or more R groups form a fusedaromatic or heteroaromatic group.

The disclosed photoactive compounds and materials are useful in visiblytransparent photovoltaic devices, such as a visibly transparentphotovoltaic device that comprises a visibly transparent substrate; afirst visibly transparent electrode coupled to the visibly transparentsubstrate; a first visibly transparent active layer coupled to the firstvisibly transparent electrode, such as a first visibly transparentphotoactive layer that comprises a photoactive compound described hereinexhibiting a greater maximum near-infrared absorption strength thanmaximum visible absorption strength; a second visibly transparentphotoactive layer coupled to the first visibly transparent photoactivelayer and exhibiting greater maximum ultraviolet absorption strength ormaximum near-infrared absorption strength than maximum visibleabsorption strength; and a second visibly transparent electrode coupledto the second visibly transparent photoactive layer.

As will be described below in further detail, the photoactive compoundsdescribed herein may be useful as electron donor materials or electronacceptor materials. In embodiments, the second visibly transparentphotoactive layer may include a second photoactive compound which may beuseful as the opposite material, such as an electron acceptor materialwhen the photoactive material of the first visibly transparentphotoactive layer is an electron donor material or as an electron donormaterial when the photoactive material of the first visibly transparentphotoactive layer is an electron acceptor material. The photoactivecompounds used in a photoactive layer may have any suitableconcentration. In some embodiments, a photoactive compound is present asa layer of substantially pure form. In some embodiments, however,photoactive layers may be present as mixed forms, such as where multiplephotoactive compounds are present, such as an electron acceptor materialand an electron donor material. Different photoactive materials may haveany suitable concentration or concentration ratios to achievephotovoltaic generation in the devices described herein. Optionally, thefirst and second visibly transparent photoactive layers independentlyhave thicknesses of about 1 nm to about 300 nm. Optionally, the firstvisibly transparent photoactive layer and the second visibly transparentphotoactive layer correspond to separate, mixed, or partially mixedlayers.

The other layers used in the visibly transparent photovoltaic devicesmay exhibit suitable compositions and properties for operation of thetransparent photovoltaic device. For example, various visiblytransparent substrates may be suitable, such as those composingtransparent glasses, transparent polymers, etc. In some embodiments, thevisibly transparent substrate may be transparent to near-infrared light(i.e., light with a wavelength greater than 650 nm) and/or ultravioletlight (i.e., light with a wavelength less than 450 nm). In this way, thevisibly transparent substrate may not absorb near-infrared and/orultraviolet light that would be suitable for photovoltaic energygeneration by the visibly transparent photovoltaic devices. In someembodiments, however, the visibly transparent substrate may absorbinfrared and/or ultraviolet light, which may be useful, for example, forconfigurations where the visibly transparent substrate serves to blockexcess infrared or ultraviolet radiation incident radiation afterpassing through the photoactive layer(s) to prevent or reduce overallultraviolet and/or infrared transmission. Useful visibly transparentsubstrates include, but are not limited to, those having thicknesses ofabout 50 nm to about 30 mm.

Example visibly transparent electrodes include those comprising indiumtin oxide or thin transparent films of conductive elements, such ascopper, gold, silver, aluminum, etc. In cases where the visiblytransparent electrodes comprise conductive elements, the thickness maybe such that even though the conductive element may be opaque in thebulk, when used as a thin film the conductive element may still allowfor transmission of visible light. Useful visibly transparent electrodesinclude, but are not limited to, those having thicknesses of about 1 nmto about 500 nm.

Other layers may be present in the visibly transparent photovoltaicdevices described herein. For example, a visibly transparentphotovoltaic device may optionally comprise one or more buffer layers,such as a first a first buffer layer disposed between the first visiblytransparent electrode and the first visibly transparent photoactivelayer and/or a second buffer layer disposed between the second visiblytransparent photoactive layer and the second visibly transparentelectrode. Useful buffer layers may serve a variety of purposes andinclude various compositions. For example, in some cases, a buffer layermay comprise a photoactive material or compound described herein.Optionally, useful buffer layers have thicknesses of about 1 nm to about500 nm.

In another aspect, methods are provided, such as methods of makingvisibly transparent photoactive materials and methods of making visiblytransparent photovoltaic devices. In an embodiment, a method of thisaspect comprises providing a transparent substrate; optionally formingone or more optical layers on the transparent substrate; forming a firsttransparent electrode; optionally forming a first buffer layer on thefirst transparent electrode; forming one or more photoactive layers,such as including a photoactive compound described herein; optionallyforming a second buffer layer; forming a second transparent electrode;and optionally forming one or more optical layers on the secondtransparent electrode. Optionally, one or more buffer layers include aphotoactive compound described herein. In some embodiments, forming oneor more layers may include making a photoactive compound and depositingthe photoactive compound. Examples of making various photoactivecompounds provided herein are described below.

Variations on these different steps will be appreciated in view of thepresent disclosure. For example, the photoactive layers may be providedas separate or mixed layers including electron acceptor and electrondonor materials. Different optical layers may be used, for example, suchas index matching layers, antireflection layers, etc.

Various techniques may be useful for forming one or more layers,including vacuum deposition techniques and solution depositiontechniques. In one embodiment, thermal evaporation is used fordepositing a photoactive compound as part of a photoactive layer. Itwill be appreciated that various patterning techniques may be useful inmethods of making a visibly transparent photovoltaic device. Forexample, visibly transparent electrodes may be optionally patterned,visibly active photoactive layers may be optionally patterned, bufferlayers may be optionally patterned, optical layers may be patterned,etc. Suitable patterning techniques may include, are not limited to,patterning techniques involving lithography, masking, lift-off, etching,etc.

These and other embodiments and aspects of the invention along with manyof its advantages and features are described in more detail inconjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified schematic diagram illustrating a visiblytransparent photovoltaic device according to an embodiment of thepresent invention.

FIG. 1B provides an overview of various configurations of photoactivelayer(s) in visibly transparent photovoltaic devices according toembodiments of the present invention.

FIG. 2 is simplified plot illustrating the solar spectrum, human eyesensitivity, and exemplary transparent photovoltaic device absorption asa function of wavelength.

FIG. 3 is a simplified energy level diagram for a visibly transparentphotovoltaic device according to an embodiment of the present invention.

FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D provide plots showing exampleabsorption profiles for different electron acceptor and donorconfigurations, which can comprise the photoactive layers.

FIG. 5 provides an overview of a method of making visibly transparentphotovoltaic devices according to embodiments of the present invention.

FIG. 6 provides a plot showing general absorption characteristics anddonor/acceptor behavior for different classes of organic molecules.

FIG. 7A provides an overview of an example synthetic route for formationof visibly transparent photoactive compounds.

FIG. 7B provides an overview of an example synthetic route for formationof visibly transparent photoactive compounds.

FIG. 8 provides data showing normalized absorbance spectra of differentvisibly transparent photoactive compounds in solution.

FIG. 9 provides data showing example current density as a function ofvoltage for an example photovoltaic cell.

FIG. 10 provides data showing example current density as a function ofvoltage for an example photovoltaic cell.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present disclosure relates to visibly transparent photoactivecompounds and visibly transparent photovoltaic devices incorporatingvisibly transparent photoactive compounds. The visibly transparentphotoactive compounds absorb light more strongly in the near-infraredregion and less strongly in the visible region, permitting their use invisibly transparent photovoltaic devices. The disclosed visiblytransparent photovoltaic devices include visibly transparent electrodeswith visibly transparent photoactive materials positioned between thevisibly transparent electrodes.

The disclosed visibly transparent photoactive compounds include metaldithiolate dyes. Example visibly transparent photoactive compoundsinclude metal dithiolate complexes including organic or heterorganicsubstituents that may be functionalized to modulate the absorption bythe compounds. For example, the disclosed photoactive compounds share acommon structural features of

where the wavy lines represent positions at which the structure isconnected to another part of the compound, which may be parts of thesame compound (i.e., a ring or fused ring structure) or differentsubstructures. In addition to the metal dithiolate components, othersubstructures of the visibly transparent photoactive compounds may beorganic or heterorganic in nature. For example, visibly transparentcompounds may include aromatic or heteroaromatic species, cyclic orheterocyclic species, each of which may be functionalized in order toachieve suitable optical and/or physical properties.

In some embodiments, for purification of the disclosed visiblytransparent photoactive compounds, a very high molecular weight, e.g.,about 500 amu or higher, about 550 amu or higher, about 600 amu orhigher, about 650 amu or higher, about 700 amu or higher, about 750 amuor higher, about 800 amu or higher, or between 500 amu and 2000 amu, maybe undesirable, as compounds with very high molecular weights may havelimited volatilities, and methods of making and purifying visiblytransparent photoactive compounds may employ an evaporation orsublimation-based purification method. In addition, the visiblytransparent photoactive compounds may be deposited into a visiblytransparent photovoltaic device using a thermal evaporation techniqueand compounds of very high molecular weight may be difficult to depositusing thermal evaporation. In various embodiments, the visiblytransparent photoactive compounds described herein have a molecularweight of 200 amu to 700 amu, less than or about 700 amu, less than orabout 650 amu, less than or about 600 amu, less than or about 550 amu,less than or about 500 amu, less than or about 450 amu, less than orabout 400 amu, less than or about 350 amu, less than or about 300 amu,less than or about 250 amu, or less than or about 200 amu.

To achieve desired optical properties, visibly transparent photoactivecompounds may exhibit a molecular electronic structure where photons ofnear-infrared light are absorbed, which results in promotion of anelectron to a higher molecular orbital, with an energy differencematching that of the absorbed photon. Compounds exhibiting extendedaromaticity or extended conjugation are beneficial, as compounds withextended aromaticity or extended conjugation may exhibit electronicabsorption with energies matching that of near-infrared and/orultraviolet photons. In some cases, however, extended aromaticity orextended conjugation may result in absorption in the visible band (i.e.,between about 450 nm and about 650 nm). In addition to conjugation andaromaticity, absorption features may be modulated by inclusion ofheteroatoms in the organic structure of the visibly transparentphotoactive compounds, such as nitrogen or sulfur atoms. Additionally oralternatively, absorption features may be modulated by the presence andpositions of electron donating or electron withdrawing groups, such ashalogen atoms, alkyl groups, alkoxy groups, etc. bonded to a core- orsub-structure of the visibly transparent photoactive compounds. Further,absorption features may optionally be modulated by the presence ofelectron donor groups or electron acceptor group within a photoactivecompound. Additionally or alternatively, absorption features may bemodulated by the identity of the central metal atom.

In general the terms and phrases used herein have their art-recognizedmeaning, which can be found by reference to standard texts, journalreferences and contexts known to those skilled in the art. The followingdefinitions are provided to clarify their specific use in the context ofthe invention.

As used herein, “maximum absorption strength” refers to the largestabsorption value in a particular spectral region, such as theultraviolet band (200 nm to 450 nm or 280 nm to 450 nm), the visibleband (450 nm to 650 nm), or the near-infrared band (650 nm to 1400 nm).In some examples, a maximum absorption strength may correspond to anabsorption strength of an absorption feature that is a local or absolutemaximum, such as an absorption band or peak, and may be referred to as apeak absorption. In some examples, a maximum absorption strength in aparticular band may not correspond to a local or absolute maximum butmay instead correspond to the largest absorption value in the particularband. Such a configuration may occur, for example, when an absorptionfeature spans multiple bands (e.g., visible and near-infrared), and theabsorption values from the absorption feature that occur within thevisible band are smaller than those occurring within the near-infraredband, such as when the peak of the absorption feature is located withinthe near-infrared band but a tail of the absorption feature extends tothe visible band. In some embodiments, a visibly transparent photoactivecompound described herein may having an absorption peak at a wavelengthgreater than about 650 nanometers (i.e., in the near-infrared), and thevisibly transparent photoactive material's absorption peak may begreater than the visibly transparent photoactive material's absorptionat any wavelength between about 450 and 650 nanometers.

In an embodiment, disclosed compositions or compounds are isolated orpurified. In an embodiment, an isolated or purified compound is at leastpartially isolated or purified as would be understood in the art. In anembodiment, a disclosed composition or compound has a chemical purity of90%, optionally for some applications 95%, optionally for someapplications 99%, optionally for some applications 99.9%, optionally forsome applications 99.99%, and optionally for some applications 99.999%pure.

Compounds disclosed herein optionally contain one or more ionizablegroups. Ionizable groups include groups from which a proton can beremoved (e.g., —COOH) or added (e.g., amines) and groups which can bequaternized (e.g., amines). All possible ionic forms of such moleculesand salts thereof are intended to be included individually in thedisclosure herein. With regard to salts of the compounds describedherein, it will be appreciated that a wide variety of availablecounter-ions may be selected that are appropriate for preparation ofsalts of this invention for a given application. In specificapplications, the selection of a given anion or cation for preparationof a salt can result in increased or decreased solubility of that salt.

The disclosed compounds optionally contain one or more chiral centers.Accordingly, this disclosure includes racemic mixtures, diastereomers,enantiomers, tautomers and mixtures enriched in one or morestereoisomer. Disclosed compounds including chiral centers encompass theracemic forms of the compound as well as the individual enantiomers andnon-racemic mixtures thereof.

As used herein, the terms “group” and “moiety” may refer to a functionalgroup of a chemical compound. Groups of the disclosed compounds refer toan atom or a collection of atoms that are a part of the compound. Groupsof the disclosed compounds may be attached to other atoms of thecompound via one or more covalent bonds. Groups may also becharacterized with respect to their valence state. The presentdisclosure includes groups characterized as monovalent, divalent,trivalent, etc. valence states. In embodiments, the term “substituent”may be used interchangeably with the terms “group” and “moiety.”

As is customary and well known in the art, hydrogen atoms in chemicalformulas disclosed herein are not always explicitly shown, for example,hydrogen atoms bonded to the carbon atoms of aliphatic, aromatic,alicyclic, carbocyclic and/or heterocyclic rings are not alwaysexplicitly shown in the formulas recited. The structures providedherein, for example in the context of the description of any specificformulas and structures recited, are intended to convey the chemicalcomposition of disclosed compounds of methods and compositions. It willbe appreciated that the structures provided do not indicate the specificpositions of atoms and bond angles between atoms of these compounds.

As used herein, the terms “alkylene” and “alkylene group” are usedsynonymously and refer to a divalent group derived from an alkyl groupas defined herein. The present disclosure includes compounds having oneor more alkylene groups. Alkylene groups in some compounds function asattaching and/or spacer groups. Disclosed compounds optionally includesubstituted and/or unsubstituted C₁-C₂₀ alkylene, C₁-C₁₀ alkylene andC₁-C₅ alkylene groups.

As used herein, the terms “cycloalkylene” and “cycloalkylene group” areused synonymously and refer to a divalent group derived from acycloalkyl group as defined herein. The present disclosure includescompounds having one or more cycloalkylene groups. Cycloalkyl groups insome compounds function as attaching and/or spacer groups. Disclosedcompounds optionally include substituted and/or unsubstituted C₃-C₂₀cycloalkylene, C₃-C₁₀ cycloalkylene and C₃-C₅ cycloalkylene groups.

As used herein, the terms “arylene” and “arylene group” are usedsynonymously and refer to a divalent group derived from an aryl group asdefined herein. The present disclosure includes compounds having one ormore arylene groups. In some embodiments, an arylene is a divalent groupderived from an aryl group by removal of hydrogen atoms from twointra-ring carbon atoms of an aromatic ring of the aryl group. Arylenegroups in some compounds function as attaching and/or spacer groups.Arylene groups in some compounds function as chromophore, fluorophore,aromatic antenna, dye and/or imaging groups. Disclosed compoundsoptionally include substituted and/or unsubstituted C₃-C₃₀ arylene,C₃-C₂₀ arylene, C₃-C₁₀ arylene and C₁-C₅ arylene groups.

As used herein, the terms “heteroarylene” and “heteroarylene group” areused synonymously and refer to a divalent group derived from aheteroaryl group as defined herein. The present disclosure includescompounds having one or more heteroarylene groups. In some embodiments,a heteroarylene is a divalent group derived from a heteroaryl group byremoval of hydrogen atoms from two intra-ring carbon atoms or intra-ringnitrogen atoms of a heteroaromatic or aromatic ring of the heteroarylgroup. Heteroarylene groups in some compounds function as attachingand/or spacer groups. Heteroarylene groups in some compounds function aschromophore, aromatic antenna, fluorophore, dye and/or imaging groups.Disclosed compounds optionally include substituted and/or unsubstitutedC₃-C₃₀ heteroarylene, C₃-C₂₀ heteroarylene, C₁-C₁₀ heteroarylene andC₃-C₅ heteroarylene groups.

As used herein, the terms “alkenylene” and “alkenylene group” are usedsynonymously and refer to a divalent group derived from an alkenyl groupas defined herein. The present disclosure includes compounds having oneor more alkenylene groups. Alkenylene groups in some compounds functionas attaching and/or spacer groups. Disclosed compounds optionallyinclude substituted and/or unsubstituted C₂-C₂₀ alkenylene, C₂-C₁₀alkenylene and C₂-C₅ alkenylene groups.

As used herein, the terms “cylcoalkenylene” and “cylcoalkenylene group”are used synonymously and refer to a divalent group derived from acylcoalkenyl group as defined herein. The present disclosure includescompounds having one or more cylcoalkenylene groups. Cycloalkenylenegroups in some compounds function as attaching and/or spacer groups.Disclosed compounds optionally include substituted and/or unsubstitutedC₃-C₂₀ cylcoalkenylene, C₃-C₁₀ cylcoalkenylene and C₃-C₅ cylcoalkenylenegroups.

As used herein, the terms “alkynylene” and “alkynylene group” are usedsynonymously and refer to a divalent group derived from an alkynyl groupas defined herein. The present disclosure includes compounds having oneor more alkynylene groups. Alkynylene groups in some compounds functionas attaching and/or spacer groups. Disclosed compounds optionallyinclude substituted and/or unsubstituted C₂-C₂₀ alkynylene, C₂-C₁₀alkynylene and C₂-C₅ alkynylene groups.

As used herein, the term “halo” refers to a halogen group, such as afluoro (—F), chloro (—Cl), bromo (—Br), or iodo (—I).

The term “heterocyclic” refers to ring structures containing at leastone other kind of atom, in addition to carbon, in the ring. Examples ofsuch atoms include sulfur, selenium, tellurium, nitrogen, phosphorus,silicon, germanium, boron, aluminum, and a transition metal. Examples ofheterocyclic rings include, but are not limited to, pyrrolidinyl,piperidinyl, imidazolidinyl, tetrahydrofuranyl, tetrahydrothienyl,furanyl, thienyl, pyridyl, quinolyl, isoquinolyl, pyridazinyl,pyrazinyl, indolyl, imidazolyl, oxazolyl, thiazolyl, pyrazolyl,pyridinyl, benzoxadiazolyl, benzothiadiazolyl, triazolyl and tetrazolylgroups. Atoms of heterocyclic rings can be bonded to a wide range ofother atoms and functional groups, for example, provided assubstituents.

The term “carbocyclic” refers to ring structures containing only carbonatoms in the ring. Carbon atoms of carbocyclic rings can be bonded to awide range of other atoms and functional groups, for example, providedas substituents.

The term “alicyclic” refers to a ring that is not an aromatic ring.Alicyclic rings include both carbocyclic and heterocyclic rings.

The term “aliphatic” refers to non-aromatic hydrocarbon compounds andgroups. Aliphatic groups generally include carbon atoms covalentlybonded to one or more other atoms, such as carbon and hydrogen atoms.Aliphatic groups may, however, include a non-carbon atom, such as anoxygen atom, a nitrogen atom, a sulfur atom, etc., in place of a carbonatom. Non-substituted aliphatic groups include only hydrogensubstituents. Substituted aliphatic groups include non-hydrogensubstituents, such as halo groups and other substituents describedherein. Aliphatic groups can be straight chain, branched, or cyclic.Aliphatic groups can be saturated, meaning only single bonds joinadjacent carbon (or other) atoms. Aliphatic groups can be unsaturated,meaning one or more double bonds or triple bonds join adjacent carbon(or other) atoms.

Alkyl groups include straight-chain, branched and cyclic alkyl groups.Alkyl groups include those having from 1 to 30 carbon atoms. Alkylgroups include small alkyl groups having 1 to 3 carbon atoms. Alkylgroups include medium length alkyl groups having from 4-10 carbon atoms.Alkyl groups include long alkyl groups having more than 10 carbon atoms,particularly those having 10-30 carbon atoms. The term cycloalkylspecifically refers to an alky group having a ring structure such asring structure comprising 3-30 carbon atoms, optionally 3-20 carbonatoms and optionally 3-10 carbon atoms, including an alkyl group havingone or more rings. Cycloalkyl groups include those having a 3-, 4-, 5-,6-, 7-, 8-, 9- or 10-member carbon ring(s) and particularly those havinga 3-, 4-, 5-, 6-, or 7-member ring(s). The carbon rings in cycloalkylgroups can also carry alkyl groups. Cycloalkyl groups can includebicyclic and tricycloalkyl groups. Alkyl groups are optionallysubstituted. Substituted alkyl groups include, among others, those whichare substituted with aryl groups, which in turn can be optionallysubstituted. Specific alkyl groups include methyl, ethyl, n-propyl,iso-propyl, cyclopropyl, n-butyl, s-butyl, t-butyl, cyclobutyl,n-pentyl, branched-pentyl, cyclopentyl, n-hexyl, branched hexyl, andcyclohexyl groups, all of which are optionally substituted. Substitutedalkyl groups include fully-halogenated or semi-halogenated alkyl groups,such as alkyl groups having one or more hydrogens replaced with one ormore fluorine atoms, chlorine atoms, bromine atoms and/or iodine atoms.Substituted alkyl groups include fully-fluorinated or semi-fluorinatedalkyl groups, such as alkyl groups having one or more hydrogens replacedwith one or more fluorine atoms.

An alkoxy group is an alkyl group that has been modified by linkage tooxygen and can be represented by the formula R—O and can also bereferred to as an alkyl ether group. Examples of alkoxy groups include,but are not limited to, methoxy, ethoxy, propoxy, butoxy and heptoxy.Alkoxy groups include substituted alkoxy groups wherein the alky portionof the groups is substituted as provided herein in connection with thedescription of alkyl groups. As used herein MeO— refers to CH₃O—.

Alkenyl groups include straight-chain, branched and cyclic alkenylgroups. Alkenyl groups include those having 1, 2 or more double bondsand those in which two or more of the double bonds are conjugated doublebonds. Alkenyl groups include those having from 2 to 20 carbon atoms.Alkenyl groups include small alkenyl groups having 2 to 4 carbon atoms.Alkenyl groups include medium length alkenyl groups having from 5-10carbon atoms. Alkenyl groups include long alkenyl groups having morethan 10 carbon atoms, particularly those having 10-20 carbon atoms.Cycloalkenyl groups include those in which a double bond is in the ringor in an alkenyl group attached to a ring. The term cycloalkenylspecifically refers to an alkenyl group having a ring structure,including an alkenyl group having a 3-, 4-, 5-, 6-, 7-, 8-, 9- or10-member carbon ring(s) and particularly those having a 3-, 4-, 5-, 6-or 7-member ring(s). The carbon rings in cycloalkenyl groups can alsocarry alkyl groups. Cycloalkenyl groups can include bicyclic andtricyclic alkenyl groups. Alkenyl groups are optionally substituted.Substituted alkenyl groups include among others those which aresubstituted with alkyl or aryl groups, which groups in turn can beoptionally substituted. Specific alkenyl groups include ethenyl,prop-1-enyl, prop-2-enyl, cycloprop-1-enyl, but-1-enyl, but-2-enyl,cyclobut-1-enyl, cyclobut-2-enyl, pent-1-enyl, pent-2-enyl, branchedpentenyl, cyclopent-1-enyl, hex-1-enyl, branched hexenyl, cyclohexenyl,all of which are optionally substituted. Substituted alkenyl groupsinclude fully-halogenated or semi-halogenated alkenyl groups, such asalkenyl groups having one or more hydrogens replaced with one or morefluorine atoms, chlorine atoms, bromine atoms and/or iodine atoms.Substituted alkenyl groups include fully-fluorinated or semi-fluorinatedalkenyl groups, such as alkenyl groups having one or more hydrogen atomsreplaced with one or more fluorine atoms.

Aryl groups include groups having one or more 5-, 6- or 7-memberaromatic and/or heterocyclic aromatic rings. The term heteroarylspecifically refers to aryl groups having at least one 5-, 6- or7-member heterocyclic aromatic rings. Aryl groups can contain one ormore fused aromatic and heteroaromatic rings or a combination of one ormore aromatic or heteroaromatic rings and one or more non-aromatic ringsthat may be fused or linked via covalent bonds. Heterocyclic aromaticrings can include one or more N, O, or S atoms in the ring, amongothers. Heterocyclic aromatic rings can include those with one, two orthree N atoms, those with one or two O atoms, and those with one or twoS atoms, or combinations of one or two or three N, O or S atoms, amongothers. Aryl groups are optionally substituted. Substituted aryl groupsinclude among others those which are substituted with alkyl or alkenylgroups, which groups in turn can be optionally substituted. Specificaryl groups include phenyl, biphenyl groups, pyrrolidinyl,imidazolidinyl, tetrahydrofuranyl, tetrahydrothienyl, furanyl, thienyl,pyridyl, quinolyl, isoquinolyl, pyridazinyl, pyrazinyl, indolyl,imidazolyl, oxazolyl, thiazolyl, pyrazolyl, pyridinyl, benzoxadiazolyl,benzothiadiazolyl, and naphthyl groups, all of which are optionallysubstituted. Substituted aryl groups include fully halogenated orsemihalogenated aryl groups, such as aryl groups having one or morehydrogens replaced with one or more fluorine atoms, chlorine atoms,bromine atoms and/or iodine atoms. Substituted aryl groups include fullyfluorinated or semifluorinated aryl groups, such as aryl groups havingone or more hydrogens replaced with one or more fluorine atoms. Arylgroups include, but are not limited to, aromatic group-containing orheterocyclic aromatic group-containing groups corresponding to any oneof the following: benzene, naphthalene, naphthoquinone, diphenylmethane,fluorene, anthracene, anthraquinone, phenanthrene, tetracene,tetracenedione, pyridine, quinoline, isoquinoline, indoles, isoindole,pyrrole, imidazole, oxazole, thiazole, pyrazole, pyrazine, pyrimidine,purine, benzimidazole, furans, benzofuran, dibenzofuran, carbazole,acridine, acridone, phenanthridine, thiophene, benzothiophene,dibenzothiophene, xanthene, xanthone, flavone, coumarin, azulene oranthracycline. As used herein, a group corresponding to the groupslisted above expressly includes an aromatic or heterocyclic aromaticgroup, including monovalent, divalent and polyvalent groups, of thearomatic and heterocyclic aromatic groups listed herein are provided ina covalently bonded configuration in the compounds of the invention atany suitable point of attachment. In embodiments, aryl groups containbetween 5 and 30 carbon atoms. In embodiments, aryl groups contain onearomatic or heteroaromatic six-membered ring and one or more additionalfive- or six-membered aromatic or heteroaromatic ring. In embodiments,aryl groups contain between five and eighteen carbon atoms in the rings.Aryl groups optionally have one or more aromatic rings or heterocyclicaromatic rings having one or more electron donating groups, electronwithdrawing groups and/or targeting ligands provided as substituents.

Arylalkyl and alkylaryl groups are alkyl groups substituted with one ormore aryl groups wherein the alkyl groups optionally carry additionalsubstituents and the aryl groups are optionally substituted. Specificalkylaryl groups are phenyl-substituted alkyl groups, e.g., phenylmethylgroups. Alkylaryl and arylalkyl groups are alternatively described asaryl groups substituted with one or more alkyl groups wherein the alkylgroups optionally carry additional substituents and the aryl groups areoptionally substituted. Specific alkylaryl groups are alkyl-substitutedphenyl groups such as methylphenyl. Substituted arylalkyl groups includefully-halogenated or semi-halogenated arylalkyl groups, such asarylalkyl groups having one or more alkyl and/or aryl groups having oneor more hydrogens replaced with one or more fluorine atoms, chlorineatoms, bromine atoms and/or iodine atoms.

As to any of the groups described herein which contain one or moresubstituents, it is understood that such groups do not contain anysubstitution or substitution patterns which are sterically impracticaland/or synthetically non-feasible. In addition, the disclosed compoundsinclude all stereochemical isomers arising from the substitution ofthese compounds. Optional substitution of alkyl groups includessubstitution with one or more alkenyl groups, aryl groups or both,wherein the alkenyl groups or aryl groups are optionally substituted.Optional substitution of alkenyl groups includes substitution with oneor more alkyl groups, aryl groups, or both, wherein the alkyl groups oraryl groups are optionally substituted. Optional substitution of arylgroups includes substitution of the aryl ring with one or more alkylgroups, alkenyl groups, or both, wherein the alkyl groups or alkenylgroups are optionally substituted.

Optional substituents for any alkyl, alkenyl and aryl group includessubstitution with one or more of the following substituents, amongothers:

halogen, including fluorine, chlorine, bromine or iodine;

pseudohalides, including —CN;

—COOR where R is a hydrogen or an alkyl group or an aryl group or, morespecifically, where R is a methyl, ethyl, propyl, butyl, or phenylgroup, all of which are optionally substituted;

—COR where R is a hydrogen or an alkyl group or an aryl group or, morespecifically, where R is a methyl, ethyl, propyl, butyl, or phenylgroup, all of which are optionally substituted;

—CON(R)₂ where each R, independently of each other R, is a hydrogen oran alkyl group or an aryl group or, more specifically, where R is amethyl, ethyl, propyl, butyl, or phenyl group, all of which areoptionally substituted; and where R and R can form a ring which cancontain one or more double bonds and can contain one or more additionalcarbon atoms;

—OCON(R)₂ where each R, independently of each other R, is a hydrogen oran alkyl group or an aryl group and, more specifically, where R is amethyl, ethyl, propyl, butyl, or phenyl group, all of which areoptionally substituted; and where R and R can form a ring which cancontain one or more double bonds and can contain one or more additionalcarbon atoms;

—N(R)₂ where each R, independently of each other R, is a hydrogen, or analkyl group, or an acyl group or an aryl group or, more specifically,where R is a methyl, ethyl, propyl, butyl, phenyl or acetyl group, allof which are optionally substituted; and where R and R can form a ringwhich can contain one or more double bonds and can contain one or moreadditional carbon atoms;

—SR, where R is hydrogen or an alkyl group or an aryl group or, morespecifically, where R is hydrogen, methyl, ethyl, propyl, butyl, or aphenyl group, all of which are optionally substituted;

—SO₂R, or —SOR where R is an alkyl group or an aryl group or, morespecifically, where R is a methyl, ethyl, propyl, butyl, or phenylgroup, all of which are optionally substituted;

—OCOOR where R is an alkyl group or an aryl group;

—SO₂N(R)₂ where each R, independently of each other R, is a hydrogen, analkyl group, or an aryl group, all of which are optionally substituted,and wherein R and R can form a ring which can contain one or more doublebonds and can contain one or more additional carbon atoms;

—OR where R is H, an alkyl group, an aryl group, or an acyl group, allof which are optionally substituted. In a particular example R can be anacyl, yielding —OCOR″ where R″ is a hydrogen or an alkyl group or anaryl group and more specifically where R″ is methyl, ethyl, propyl,butyl, or phenyl groups, all of which are optionally substituted.

Specific substituted alkyl groups include haloalkyl groups, particularlytrihalomethyl groups and specifically trifluoromethyl groups. Specificsubstituted aryl groups include mono-, di-, tri, tetra- andpenta-halo-substituted phenyl groups; mono-, di-, tri-, tetra-, penta-,hexa-, and hepta-halo-substituted naphthalene groups; 3- or4-halo-substituted phenyl groups, 3- or 4-alkyl-substituted phenylgroups, 3- or 4-alkoxy-substituted phenyl groups, 3- or4-RCO-substituted phenyl, 5- or 6-halo-substituted naphthalene groups.More specifically, substituted aryl groups include acetylphenyl groups,particularly 4-acetylphenyl groups; fluorophenyl groups, particularly3-fluorophenyl and 4-fluorophenyl groups; chlorophenyl groups,particularly 3-chlorophenyl and 4-chlorophenyl groups; methylphenylgroups, particularly 4-methylphenyl groups; and methoxyphenyl groups,particularly 4-methoxyphenyl groups.

FIG. 1A is a simplified schematic diagram illustrating a visiblytransparent photovoltaic device according to an embodiment of thepresent invention. As illustrated in FIG. 1A, the visibly transparentphotovoltaic device 100 includes a number of layers and elementsdiscussed more fully below. As discussed in relation to FIG. 2 , visiblytransparent indicates that the photovoltaic device absorbs opticalenergy at wavelengths outside the visible wavelength band of 450 nm to650 nm, for example, while substantially transmitting visible lightinside the visible wavelength band. As illustrated in FIG. 1A, UV and/orNIR light is absorbed in the layers and elements of the photovoltaicdevice while visible light is transmitted through the device. Thus, thediscussion of transparency provided herein should be understood asvisible transparency.

Substrate 105, which can be glass or other visibly transparent materialsproviding sufficient mechanical support to the other layers andstructures illustrated, supports optical layers 110 and 112. Theseoptical layers can provide a variety of optical properties, includingantireflection (AR) properties, wavelength selective reflection ordistributed Bragg reflection properties, index matching properties,encapsulation, or the like. Optical layers may advantageously be visiblytransparent. An additional optical layer 114 can be utilized, forexample, as an AR coating, an index matching later, a passive infraredor ultraviolet absorption layer, etc. Optionally, optical layers may betransparent to ultraviolet and/or near-infrared light or transparent toat least a subset of wavelengths in the ultraviolet and/or near-infraredbands. Depending on the configuration, additional optical layer 114 mayalso be a passive visible absorption layer. Example substrate materialsinclude various glasses and rigid or flexible polymers. Multilayersubstrates may also be utilized. Substrates may have any suitablethickness to provide the mechanical support needed for the other layersand structures, such as, for example, thicknesses from 1 mm to 20 mm. Insome cases, the substrate may be or comprise an adhesive film to allowapplication of the visibly transparent photovoltaic device 100 toanother structure, such as a window pane, display device, etc.

It will be appreciated that, although the devices overall may exhibitvisible transparency, such as a transparency in the 450-650 nm rangegreater than 30%, greater than 40%, greater than 50%, greater than 60%,greater than 70%, or up to or approaching 100%, certain materials takenindividually may exhibit absorption in portions of the visible spectrum.Optionally, each individual material or layer in a visibly transparentphotovoltaic device has a high transparency in the visible range, suchas greater than 30% (i.e., between 30% and 100%). It will be appreciatedthat transmission or absorption may be expressed as a percentage and maybe dependent on the material's absorbance properties, a thickness orpath length through an absorbing material, and a concentration of theabsorbing material, such that a material with an absorbance in thevisible spectral region may still exhibit a low absorption or hightransmission if the path length through the absorbing material is shortand/or the absorbing material is present in low concentration.

As described herein and below, photoactive materials in variousphotoactive layers advantageously exhibit minimal absorption in thevisible region (e.g., less than 20%, less than 30%, less than 40%, lessthan 50%, less than 60%, or less than 70%), and instead exhibit highabsorption in the near-infrared and/or ultraviolet regions (e.g., anabsorption peak of greater than 50%, greater than 60%, greater than 70%,or greater than 80%). For some applications, absorption in the visibleregion may be as large as 70%. Various configurations of othermaterials, such as the substrate, optical layers, and buffer layers, mayallow these materials to provide overall visible transparency, eventhough the materials may exhibit some amount of visible absorption. Forexample, a thin film of a metal may be included in a transparentelectrode, such as a metal that exhibits visible absorption, like Ag orCu; when provided in a thin film configuration, however, the overalltransparency of the film may be high. Similarly, materials included inan optical or buffer layer may exhibit absorption in the visible range,but may be provided at a concentration or thickness where the overallamount of visible light absorption is low, providing visibletransparency.

The visibly transparent photovoltaic device 100 also includes a set oftransparent electrodes 120 and 122 with a photoactive layer 140positioned between electrodes 120 and 122. These electrodes, which canbe fabricated using ITO, thin metal films, or other suitable visiblytransparent materials, provide electrical connection to one or more ofthe various layers illustrated. For example, thin films of copper,silver, or other metals may be suitable for use as a visibly transparentelectrode, even though these metals may absorb light in the visibleband. When provided as a thin film, however, such as a film having athickness of 1 nm to 200 nm (e.g., about 5 nm, about 10 nm, about 15 nm,about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about45 nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70 nm,about 75 nm, about 80 nm, about 85 nm, about 90 nm, about 95 nm, about100 nm, about 105 nm, about 110 nm, about 115 nm, about 120 nm, about125 nm, about 130 nm, about 135 nm, about 140 nm, about 145 nm, about150 nm, about 155 nm, about 160 nm, about 165 nm, about 170 nm, about175 nm, about 180 nm, about 185 nm, about 190 nm, or about 195 nm), anoverall transmittance of the thin film in the visible band may remainhigh, such as greater than 30%, greater than 40%, greater than 50%,greater than 60%, greater than 70%, greater than 80%, or greater than90%. Advantageously, thin metal films, when used as transparentelectrodes, may exhibit lower absorption in the ultraviolet band thanother semiconducting materials that may be useful as a transparentelectrode, such as ITO, as some semiconducting transparent conductingoxides exhibit a band gap that occurs in the ultraviolet band and thusare highly absorbing or opaque to ultraviolet light. In some cases,however, an ultraviolet absorbing transparent electrode may be used,such as to screen at least a portion of the ultraviolet light fromunderlying components, as ultraviolet light may degrade certainmaterials.

A variety of deposition techniques may be used to generate a transparentelectrode, including vacuum deposition techniques, such as atomic layerdeposition, chemical vapor deposition, physical vapor deposition,thermal evaporation, sputter deposition, epitaxy, etc. Solution baseddeposition techniques, such as spin-coating, may also be used in somecases. In addition, transparent electrodes may be patterned usingtechniques known in the art of microfabrication, including lithography,lift off, etching, etc.

Buffer layers 130 and 132 and photoactive layer 140 are utilized toimplement the electrical and optical properties of the photovoltaicdevice. These layers can be layers of a single material or can includemultiple sub-layers as appropriate to the particular application. Thus,the term “layer” is not intended to denote a single layer of a singlematerial, but can include multiple sub-layers of the same or differentmaterials. In some embodiments, buffer layer 130, photoactive layer(s)140 and buffer layer 132 are repeated in a stacked configuration toprovide tandem device configurations, such as including multipleheterojunctions. In some embodiments, the photoactive layer(s) includeelectron donor materials and electron acceptor materials, also referredto as donors and acceptors. These donors and acceptors are visiblytransparent, but absorb outside the visible wavelength band to providethe photoactive properties of the device.

Useful buffer layers include those that function as electron transportlayers, electron blocking layers, hole transport layers, hole blockinglayers, exciton blocking layers, optical spacers, physical bufferlayers, charge recombination layers, or charge generation layers. Bufferlayers may exhibit any suitable thickness to provide the bufferingeffect desired and may optionally be present or absent. Useful bufferlayers, when present, may have thicknesses from 1 nm to 1 μm. Variousmaterials may be used as buffer layers, including fullerene materials,carbon nanotube materials, graphene materials, metal oxides, such asmolybdenum oxide, titanium oxide, zinc oxide, etc., polymers, such aspoly(3,4-ethylenedioxythiophene), polystyrene sulfonic acid,polyaniline, etc., copolymers, polymer mixtures, and small molecules,such as bathocuproine. Buffer layers may be applied using a depositionprocess (e.g., thermal evaporation) or a solution processing method(e.g., spin coating).

FIG. 1B depicts an overview of various example single junctionconfigurations for photoactive layer 140. Photoactive layer 140 mayoptionally correspond to mixed donor/acceptor (bulk heterojunction)configurations, planar donor/acceptor configurations, planar and mixeddonor/acceptor configurations, or gradient donor/acceptorconfigurations. Various materials may be used as the photoactive layers140, such as materials that absorb in the ultraviolet band or thenear-infrared band but that only absorb minimally, if at all, in thevisible band. In this way, the photoactive material may be used togenerate electron-hole pairs for powering an external circuit by way ofultraviolet and/or near-infrared absorption, leaving the visible lightrelatively unperturbed to provide visible transparency. As illustrated,photoactive layer 140 may comprise a planar heterojunction includingseparate donor and acceptor layers. Photoactive layer 140 mayalternatively comprise a planar-mixed heterojunction structure includingseparate acceptor and donor layers and a mixed donor-acceptor layer.Photoactive layer 140 may alternatively comprise a mixed heterojunctionstructure including a fully mixed acceptor-donor layer or thoseincluding a mixed donor-acceptor layer with various relativeconcentration gradients.

Photoactive layers may have any suitable thickness and may have anysuitable concentration or composition of photoactive materials toprovide a desired level of transparency and ultraviolet/near-infraredabsorption characteristics. Example thicknesses of a photoactive layermay range from about 1 nm to about 1 μm, about 1 nm to about 300 nm, orabout 1 nm to about 100 nm. In some cases, photoactive layers may bemade up of individual sub-layers or mixtures of layers to providesuitable photovoltaic power generation characteristics, as illustratedin FIG. 1B. The various configurations depicted in FIG. 1B may be usedand dependent on the particular donor and acceptor materials used inorder to provide advantageous photovoltaic power generation. Forexample, some donor and acceptor combinations may benefit fromparticular configurations, while other donor and acceptor combinationsmay benefit from other particular configurations. Donor materials andacceptor materials may be provided in any ratio or concentration toprovide suitable photovoltaic power generation characteristics. Formixed layers, the relative concentration of donors to acceptors isoptionally between about 20 to 1 and about 1 to 20. Optionally, therelative concentration of donors to acceptors is optionally betweenabout 5 to 1 and about 1 to 5. Optionally, donors and acceptors arepresent in a 1 to 1 ratio.

It will be appreciated that, in various embodiments, visibly transparentphotovoltaic device 100 comprises transparent electrode 120, photoactivelayer 140 and transparent electrode 122, and that any one or more ofsubstrate 105, optical layers 110, 112, and 114, and buffer layers 130and 132 may be optionally included or excluded.

As described more fully herein, embodiments of the present inventionemploy visibly transparent metal dithiolate compounds for one or more ofthe buffer layers, optical layers, and/or the photoactive layers. Thesecompounds can include suitably functionalized versions for modificationof the electrical and/or optical properties of the core structure. As anexample, the disclosed compounds can include functional groups thatdecrease the absorption properties in the visible wavelength bandbetween 450 nm to 650 nm and increase the absorption properties in theNIR band at wavelengths greater than 650 nm.

As an example, the disclosed visibly transparent photoactive compoundsare useful as an electron donor photoactive material, in someembodiments, and may be paired with suitable electron acceptorphotoactive materials in order to provide a useful photoactive layer inthe photovoltaic device. Optionally, the disclosed visibly transparentphotoactive compounds are useful as an electron acceptor photoactivematerial, and may be paired with suitable electron donor photoactivematerials in order to provide a useful photoactive layer in thephotovoltaic device. Example donor and acceptor materials are describedin U.S. Provisional Application Nos. 62/521,154, 62/521,158, 62/521,160,62/521,211, 62/521,214, and 62/521,224, each filed on Jun. 16, 2017,which are hereby incorporated by reference in their entireties.

In embodiments, the chemical structure of the disclosed photoactivecompounds can be functionalized with one or more directing groups, suchas electron donating groups, electron withdrawing groups, orsubstitutions about or to a core metal atom, in order to providedesirable electrical characteristics to the material. For example, insome embodiments, the photoactive compounds are functionalized withamine groups, phenol groups, alkyl groups, phenyl groups, or otherelectron donating groups to improve the ability of the material tofunction as an electron donor in a photovoltaic device. As anotherexample, in some embodiments, the photoactive compounds arefunctionalized with cyano groups, halogens, sulfonyl groups, or otherelectron withdrawing groups to improve the ability of the material tofunction as an electron acceptor in a photovoltaic device.

In embodiments, the photoactive compounds are functionalized to providedesirable optical characteristics. For example, in some embodiments, thephotoactive compounds may be functionalized with an extended conjugationto redshift the absorption profile of the material. It will beappreciated that conjugation may refer to a delocalization of pielectrons in a molecule and may be characterized by alternating singleand multiple bonds in a molecular structure. For example,functionalizations that extend the electron conjugation may includefusing one or more aromatic groups to the molecular structure of thematerial. Other functionalizations that may provide extended conjugationinclude alkene functionalization, such as by a vinyl group, aromatic orheteroaromatic functionalization, carbonyl functionalization, such as byan acyl group, sulfonyl functionalization, nitro functionalization,cyano functionalization, etc. It will be appreciated that variousmolecular functionalizations may impact both the optical and theelectrical properties of the photoactive compounds.

It will be appreciated that device function may be impacted by themorphology of the active layers in the solid state. Separation ofelectron donors and acceptors into discrete domains, with dimensions onthe scale of the exciton diffusion length and large interfacial areas,can be advantageous for achieving high device efficiency.Advantageously, the molecular framework of the photoactive materials canbe tailored to control the morphology of the materials. For example, theintroduction of functional groups as described herein can have largeimpacts to the morphology of the material in the solid state, regardlessof whether such modifications impact the energetics or electronicproperties of the material. Such morphological variations can beobserved in pure materials and when a particular material is blendedwith a corresponding donor or acceptor. Useful functionalities tocontrol morphology include, but are not limited to, addition of alkylchains, conjugated linkers, fluorinated alkanes, bulky groups (e.g.,tert-butyl, phenyl, naphthyl or cyclohexyl), as well as more complexcoupling procedures designed to force parts of the structure out of theplane of the molecule to inhibit excessive crystallization.

In embodiments, other molecular structural characteristics may providedesirable electrical and optical properties in the photoactivecompounds. For example, in some embodiments, the photoactive compoundsmay exhibit portions of the molecule that may be characterized aselectron donating while other portions of the molecule may becharacterized as electron accepting. Without wishing to be bound by anytheory, molecules including alternating electron donating and electronaccepting portions may result in redshifting the absorptioncharacteristics of the molecule as compared to similar molecules lackingalternating electron donating and electron accepting portions. Forexample, alternating electron donating and electron accepting portionsmay decrease or otherwise result in a lower energy gap between a highestoccupied molecular orbital and a lowest unoccupied molecular orbital.Organic donor and/or acceptor groups may be useful as R-groupsubstituents, such as on any aryl, aromatic, heteroaryl, heteroaromatic,alkyl, or alkenyl group, in the visibly transparent photoactivecompounds described herein.

which may be considered strong acceptor groups. Example organic acceptorgroups include:

which may be considered moderate acceptor groups. Example organicacceptor groups include:

which may be considered weaker acceptor groups. For the illustratedorganic acceptor groups, R^(A) may optionally be hydrogen or alkyl, forexample. Further, the wavy lines represent positions at which theillustrated structure is connected to another substructure.

Example organic donor groups include:

which may be considered strong donor groups. Example organic donorgroups include:

which may be considered moderate donor groups. Example organic donorgroups include:

which may be considered weaker donor groups. For the illustrated organicdonor groups, R^(D) may optionally be hydrogen or alkyl, for example.Further, the wavy lines represent positions at which the illustratedstructure is connected to another substructure.

In embodiments, the photoactive compounds may exhibit symmetricstructures, such as structures having two or more points of symmetry.Symmetric structures may include those where a core group, such as ametal dithiolate complex, is functionalized on opposite sides by thesame groups, or where two of the same core groups are fused or otherwisebonded to one another. In other embodiments, the photoactive compoundsmay exhibit asymmetric structures, such as structures having fewer thantwo points of symmetry. Asymmetric structures may include those where acore group, such as a metal dithiolate complex, is functionalized onopposite sides by different groups or where two different core groupsare fused or otherwise bonded to one another.

When the materials described herein are incorporated as a photoactivelayer in a transparent photovoltaic device as either an electron donoror electron acceptor, the layer thicknesses can be controlled to varydevice output, absorbance, or transmittance. For example, increasing thedonor or acceptor layer thickness can increase the light absorption inthat layer. In some cases, increasing a concentration of donor/acceptormaterials in a donor or acceptor layer may similarly increase the lightabsorption in that layer. However, in some embodiments, a concentrationof donor/acceptor materials may not be adjustable, such as when activematerial layers comprise pure or substantially pure layers ofdonor/acceptor materials or pure or substantially pure mixtures ofdonor/acceptor materials. Optionally, donor/acceptor materials may beprovided in a solvent or suspended in a carrier, such as a buffer layermaterial, in which case the concentration of donor/acceptor materialsmay be adjusted. In some embodiments, the donor layer concentration isselected where the current produced is maximized. In some embodiments,the acceptor layer concentration is selected where the current producedis maximized.

However, the charge collection efficiency can decrease with increasingdonor or acceptor thickness due to the increased “travel distance” forthe charge carriers. Therefore, there may be a trade-off betweenincreased absorption and decreasing charge collection efficiency withincreasing layer thickness. It can thus be advantageous to selectmaterials as described herein that have a high absorption coefficientand/or concentration to allow for increased light absorption perthickness. In some embodiments, the donor layer thickness is selectedwhere the current produced is maximized. In some embodiments, theacceptor layer thickness is selected where the current produced ismaximized.

In addition to the individual photoactive layer thicknesses formed frommaterials described herein, the thickness and composition of the otherlayers in the transparent photovoltaic device can also be selected toenhance absorption within the photoactive layers. The other layers(buffer layers, electrodes, etc.), are typically selected based on theiroptical properties (index of refraction and extinction coefficient) inthe context of the thin film device stack and resulting optical cavity.For example, a near-infrared absorbing photoactive layer can bepositioned in the peak of the optical field for the near-infraredwavelengths where it absorbs to maximize absorption and resultingcurrent produced by the device. This can be accomplished by spacing thephotoactive layer at an appropriate distance from the electrode using asecond photoactive layer and/or optical layers as spacer. A similarscheme can be used for ultraviolet absorbing photoactive layers. In manycases, the peaks of the longer wavelength optical fields will bepositioned further from the more reflective of the two transparentelectrodes compared to the peaks of the shorter wavelength opticalfields. Thus, when using separate donor and acceptor photoactive layers,the donor and acceptor can be selected to position the more redabsorbing (longer wavelength) material further from the more reflectiveelectrode and the more blue absorbing (shorter wavelength) closer to themore reflective electrode.

In some embodiments, optical layers may be included to increase theintensity of the optical field at wavelengths where the donor absorbs inthe donor layer to increase light absorption and hence, increase thecurrent produced by the donor layer. In some embodiments, optical layersmay be included to increase the intensity of the optical field atwavelengths where the acceptor absorbs in the acceptor layer to increaselight absorption and hence, increase the current produced by theacceptor layer. In some embodiments, optical layers may be used toimprove the transparency of the stack by either decreasing visibleabsorption or visible reflection. Further, the electrode material andthickness may be selected to enhance absorption outside the visiblerange within the photoactive layers, while preferentially transmittinglight within the visible range.

Optionally, enhancing spectral coverage of a visibly transparentphotovoltaic device is achieved by the use of a multi-cell series stackof visibly transparent photovoltaic devices, referred to as tandemcells, which may be included as multiple stacked instances of bufferlayer 130, photoactive layer 140, and buffer layer 132, as describedwith reference to FIG. 1A. This architecture includes more than onephotoactive layer, which are typically separated by a combination ofbuffer layer(s) and/or thin metal layers, for example. In thisarchitecture, the currents generated in each subcell flow in series tothe opposing electrodes and therefore, the net current in the cell islimited by the smallest current generated by a particular subcell, forexample. The open circuit voltage (VOC) is equal to the sum of the VOCsof the subcells. By combining sub-cells fabricated with differentdonor-acceptors pairs which absorb in different regions of the solarspectrum, a significant improvement in efficiency relative to a singlejunction cell can be achieved.

Additional description related to the materials utilized in one or moreof the buffer layers and the photoactive layers, including donor layersand/or acceptor layers, are provided below.

FIG. 2 is simplified plot illustrating the solar spectrum, human eyesensitivity, and exemplary visibly transparent photovoltaic deviceabsorption as a function of wavelength. As illustrated in FIG. 2 ,embodiments of the present invention utilize photovoltaic structuresthat have low absorption in the visible wavelength band between about450 nm and about 650 nm, but absorb in the UV and NIR bands, i.e.,outside the visible wavelength band, enabling visibly transparentphotovoltaic operation. The ultraviolet band or ultraviolet region maybe described, in embodiments, as wavelengths of light of between about200 nm and 450 nm. It will be appreciated that useful solar radiation atground level may have limited amounts of ultraviolet less than about 280nm and, thus, the ultraviolet band or ultraviolet region may bedescribed as wavelengths of light of between about 280 nm and 450 nm, insome embodiments. The near-infrared band or near-infrared region may bedescribed, in embodiments, as wavelengths of light of between about 650nm and 1400 nm. Various compositions described herein may exhibitabsorption including a NIR peak with a maximum absorption strength inthe visible region that is smaller than that in the NIR region.

FIG. 3 provides a schematic energy level diagram overview for operationof an example organic photovoltaic device, such as visibly transparentphotovoltaic device 100. For example, in such a photovoltaic device,various photoactive materials may exhibit electron donor or electronacceptor characteristics, depending on their molecular properties andthe types of materials that are used for buffer layers, electrodes, etc.As depicted in FIG. 3 , each of the donor and acceptor materials have ahighest occupied molecular orbital (HOMO) and a lowest unoccupiedmolecular orbital (LUMO). A transition of an electron from the HOMO tothe LUMO may be imparted by absorption of photons. The energy betweenthe HOMO and the LUMO (the HOMO-LUMO gap) of a material representsapproximately the energy of the optical band gap of the material. Forthe electron donor and electron acceptor materials useful with thetransparent photovoltaic devices provided herein, the HOMO-LUMO gap forthe electron donor and electron acceptor materials preferably fallsoutside the energy of photons in the visible range. For example, theHOMO-LUMO gap may be in the ultraviolet region or the near-infraredregion, depending on the photoactive materials. It will be appreciatedthat the HOMO is comparable to the valence band in conventionalconductors or semiconductors, while the LUMO is comparable to theconduction band in conventional conductors or semiconductors.

The narrow absorption spectrum of many organic molecules, such asorganic semiconductors, can make it difficult to absorb the entireabsorption spectra using a single molecular species. Therefore, electrondonor and acceptor molecules are generally paired to afford acomplementary absorption spectrum and increase spectral coverage oflight absorption. Additionally, the donor and acceptor molecules areselected such that their energy levels (HOMO and LUMO) lie favorablywith respect to one another. The difference in the LUMO level of donorand acceptor provides a driving force for dissociation of electron-holepairs (excitons) created on the donor whereas the difference in the HOMOlevels of donor and acceptors provides driving force for dissociation ofelectron-hole pairs (excitons) created on the acceptor. In someembodiments, it may be useful for the acceptor to have high electronmobility to efficiently transport electrons to an adjacent buffer layer.In some embodiments, it may be useful for the donor to have high holemobility to efficiently transport holes to the buffer layer.Additionally, in some embodiments, it may be useful to increase thedifference in the LUMO level of the acceptor and HOMO level of the donorto increase the open circuit voltage (VOC), since VOC has been shown tobe directly proportional to the difference between LUMO of the acceptorand HOMO of the donor. Such donor-acceptor pairings within thephotoactive layer may be accomplished by appropriately pairing one ofthe materials described herein with a complementary material, whichcould be a different visibly transparent photoactive compound describedherein or a completely separate material system, as described below withreference to FIG. 6 .

The buffer layer adjacent to the donor, generally referred to as theanode buffer layer or hole transport layer, is selected such that HOMOlevel or valence band (in the case of inorganic materials) of the bufferlayer is aligned in the energy landscape with the HOMO level of thedonor to transport holes from the donor to the anode (transparentelectrode). In some embodiments, it may be useful for the buffer layerto have high hole mobility. The buffer layer adjacent to the acceptor,generally referred to as the cathode buffer layer or electron transportlayer, is selected such that LUMO level or conduction band (in the caseof inorganic materials) of the buffer layer is aligned in the energylandscape with the LUMO level of the acceptor to transport electronsfrom the acceptor to the cathode (transparent electrode). In someembodiments, it may be useful for the buffer layer to have high electronmobility.

FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D provide plots showing exampleabsorption bands for different electron donor and electron acceptorconfigurations useful with visibly transparent photovoltaic devices. InFIG. 4A, the donor material exhibits absorption in the NIR, while theacceptor material exhibits absorption in the UV. FIG. 4B depicts theopposite configuration, where the donor material exhibits absorption inthe UV, while the acceptor material exhibits absorption in the NIR.

FIG. 4C depicts an additional configuration, where both the donor andacceptor materials exhibit absorption in the NIR. As illustrated in thefigures, the solar spectrum exhibits significant amounts of usefulradiation in the NIR with only relatively minor amounts in theultraviolet, making the configuration depicted in FIG. 4C useful forcapturing a large amount of energy from the solar spectrum. It will beappreciated that other embodiments are contemplated where both the donorand acceptor materials exhibit absorption in the NIR, such as depictedin FIG. 4D where the acceptor is blue shifted relative to the donor,opposite the configuration depicted in FIG. 4C, where the donor is blueshifted relative to the acceptor.

The present invention also provides methods for making visiblytransparent photovoltaic devices, such as visibly transparentphotovoltaic device 100. For example, FIG. 5 provides an overview of amethod 500 for making a visibly transparent photovoltaic device inaccordance with some embodiments. Method 500 begins at block 505, wherea transparent substrate is provided. It will be appreciated that usefultransparent substrates include visibly transparent substrates, such asglass, plastic, quartz, and the like. Flexible and rigid substrates areuseful with various embodiments. Optionally, the transparent substrateis provided with one or more optical layers preformed on top and/orbottom surfaces.

At block 510, one or more optical layers are optionally formed on orover the transparent substrate, such as on top and/or bottom surfaces ofthe transparent substrate. Optionally, the one or more optical layersare formed on other materials, such as an intervening layer or material,such as a transparent conductor. Optionally, the one or more opticallayers are positioned adjacent to and/or in contact with the visiblytransparent substrate. It will be appreciated that formation of opticallayers is optional, and some embodiments may not include optical layersadjacent to and/or in contact with the transparent substrate. Opticallayers may be formed using a variety of methods including, but notlimited to, one or more chemical deposition methods, such as plating,chemical solution deposition, spin coating, dip coating, chemical vapordeposition, plasma enhanced chemical vapor deposition, and atomic layerdeposition, or one or more physical deposition methods, such as thermalevaporation, electron beam evaporation, molecular beam epitaxy,sputtering, pulsed laser deposition, ion beam deposition, andelectrospray deposition. It will be appreciated that useful opticallayers include visibly transparent optical layers. Useful optical layersinclude those that provide one or more optical properties including, forexample, antireflection properties, wavelength selective reflection ordistributed Bragg reflection properties, index matching properties,encapsulation, or the like. Useful optical layers may optionally includeoptical layers that are transparent to ultraviolet and/or near-infraredlight. Depending on the configuration, however, some optical layers mayoptionally provide passive infrared and/or ultraviolet absorption.Optionally, an optical layer may include a visibly transparentphotoactive compound described herein.

At block 515, a transparent electrode is formed. As described above, thetransparent electrode may correspond to an indium tin oxide thin film orother transparent conducting film, such as thin metal films (e.g., Ag,Cu, etc.), multilayer stacks comprising thin metal films (e.g., Ag, Cu,etc.) and dielectric materials, or conductive organic materials (e.g.,conducting polymers, etc.). It will be appreciated that transparentelectrodes include visibly transparent electrodes. Transparentelectrodes may be formed using one or more deposition processes,including vacuum deposition techniques, such as atomic layer deposition,chemical vapor deposition, physical vapor deposition, thermalevaporation, sputter deposition, epitaxy, etc. Solution based depositiontechniques, such as spin-coating, may also be used in some cases. Inaddition, transparent electrodes may be patterned by way ofmicrofabrication techniques, such as lithography, lift off, etching,etc.

At block 520, one or more buffer layers are optionally formed, such ason the transparent electrode. Buffer layers may be formed using avariety of methods including, but not limited to, one or more chemicaldeposition methods, such as a plating, chemical solution deposition,spin coating, dip coating, chemical vapor deposition, plasma enhancedchemical vapor deposition, and atomic layer deposition, or one or morephysical deposition methods, such as thermal evaporation, electron beamevaporation, molecular beam epitaxy, sputtering, pulsed laserdeposition, ion beam deposition, and electrospray deposition. It will beappreciated that useful buffer layers include visibly transparent bufferlayers. Useful buffer layers include those that function as electrontransport layers, electron blocking layers, hole transport layers, holeblocking layers, optical spacers, physical buffer layers, chargerecombination layers, or charge generation layers. In some cases, thedisclosed visibly transparent photoactive compounds may be useful as abuffer layer material. For example, a buffer layer may optionallyinclude a visibly transparent photoactive compound described herein.

At block 525, one or more photoactive layers are formed, such as on abuffer layer or on a transparent electrode. As described above,photoactive layers may comprise electron acceptor layers and electrondonor layers or co-deposited layers of electron donors and acceptors.Useful photoactive layers include those comprising the visiblytransparent photoactive compounds described herein. Photoactive layersmay be formed using a variety of methods including, but not limited to,one or more chemical deposition methods, such as a plating, chemicalsolution deposition, spin coating, dip coating, chemical vapordeposition, plasma enhanced chemical vapor deposition, and atomic layerdeposition, or one or more physical deposition methods, such as thermalevaporation, electron beam evaporation, molecular beam epitaxy,sputtering, pulsed laser deposition, ion beam deposition, andelectrospray deposition.

In some examples, visibly transparent photoactive compounds useful forphotoactive layers may be deposited using a vacuum deposition technique,such as thermal evaporation. Vacuum deposition may take place in avacuum chamber, such as at pressures of between about 10⁻⁵ Torr andabout 10⁻⁸ Torr. In one example, vacuum deposition may take place at apressure of about 10⁻⁷ Torr. As noted above, various depositiontechniques may be applied. In some embodiments, thermal evaporation isused. Thermal evaporation may include heating a source of the material(i.e., the visibly transparent photoactive compound) to be deposited toa temperature of between 200° C. and 500° C. The temperature of thesource of material may be selected so as to achieve a thin film growthrate of between about 0.01 nm/s and about 1 nm/s. For example, a thinfilm growth rate of 0.1 nm/s may be used. These growth rates are usefulto generate thin films having thicknesses of between about 1 nm and 500nm over the course of minutes to hours. It will be appreciated thatvarious properties (e.g., the molecular weight, volatility, thermalstability) of the material being deposited may dictate or influence thesource temperature or maximum useful source temperature. For example, athermal decomposition temperature of the material being deposited maylimit the maximum temperature of the source. As another example, amaterial being deposited that is highly volatile may require a lowersource temperature to achieve a target deposition rate as compared to amaterial that is less volatile, where a higher source temperature may beneeded to achieve the target deposition rate. As the material beingdeposited is evaporated from the source, it may be deposited on asurface (e.g., substrate, optical layer, transparent electrode, bufferlayer, etc.) at a lower temperature. For example the surface may have atemperature from about 10° C. to about 100° C. In some cases, thetemperature of the surface may be actively controlled. In some cases,the temperature of the surface may not be actively controlled.

At block 530, one or more buffer layers are optionally formed, such ason the photoactive layer. The buffer layers formed at block 530 may beformed similar to those formed at block 520. It will be appreciated thatblocks 520, 525, and 530 may be repeated one or more times, such as toform a multilayer stack of materials including a photoactive layer and,optionally, various buffer layers.

At block 535, a second transparent electrode is formed, such as on abuffer layer or on a photoactive layer. Second transparent electrode maybe formed using techniques applicable to formation of first transparentelectrode at block 515.

At block 540, one or more additional optical layers are optionallyformed, such as on the second transparent electrode.

It should be appreciated that the specific steps illustrated in FIG. 5provide a particular method of making a visibly transparent photovoltaicdevice according to various embodiments of the present invention. Othersequences of steps may also be performed according to alternativeembodiments. For example, alternative embodiments of the presentinvention may perform the steps outlined above in a different order.Moreover, the individual steps illustrated in FIG. 5 may includemultiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or removed depending on the particular applications. It will beappreciated that many variations, modifications, and alternatives may beused.

Method 500 may optionally be extended to correspond to a method forgenerating electrical energy. For example, a method for generatingelectrical energy may comprise providing a visibly transparentphotovoltaic device, such as by making a visibly transparentphotovoltaic device according to method 500. Methods for generatingelectrical energy may further comprise exposing the visibly transparentphotovoltaic device to visible, ultraviolet and/or near-infrared lightto drive the formation and separation of electron-hole pairs, asdescribed above with reference to FIG. 3 , for example, for generationof electrical energy. The visibly transparent photovoltaic device mayinclude the visibly transparent photoactive compounds described hereinas photoactive materials, buffer materials, and/or optical layers.

FIG. 6 provides a plot showing general absorption characteristics anddonor/acceptor behavior for different classes of photoactive compounds.Useful visibly transparent photoactive layers in visibly transparentphotovoltaic devices may include one or more photoactive compounds ofthe different classes illustrated. For example, photoactive compoundsdescribed herein may be incorporated into a visibly transparentphotoactive layer as a donor or acceptor compound along with anotherphotoactive compound of the classes depicted in FIG. 6 . As an example,if a photoactive compound is used as an electron donor photoactivematerial, it may be paired with an electron acceptor photoactivematerial from the classes illustrated in FIG. 6 as a counterpart inorder to provide a useful photoactive layer in the photovoltaic device.Similarly, if a photoactive compound is used as an electron acceptorphotoactive material, it may be paired with an electron donorphotoactive material from the classes illustrated in FIG. 6 as acounterpart in order to provide a useful photoactive layer in thephotovoltaic device. As an example, TAPC, or a derivative thereof, maybe used as a UV absorbing electron donor. As another example, fullerenesor fullerene derivatives may optionally be used as a UV absorbingelectron acceptor. As another example, a cyanine salt may be used as anear-infrared absorbing electron donor. As another example, a moleculewith a donor-acceptor-donor structure (D-A-D) may be useful as anear-infrared absorbing electron donor photoactive material or anelectron acceptor photoactive material. As another example, a moleculewith an acceptor-donor-acceptor structure (A-D-A) may be useful as anear-infrared absorbing electron acceptor photoactive material. Exampleacceptor groups and donor groups for D-A-D and A-D-A structures aredescribed above. In some cases, the different classes illustrated mayexhibit overlapping absorption features (i.e., spectral ranges). It maybe desirable, however, to select a counterpart photoactive compound withabsorption features from a different spectral region or that do not haveoverlapping absorption features or that have different peak absorptionwavelengths.

The visibly transparent photoactive compounds, useful as photoactivematerials, buffer materials, and/or optical layers, include metaldithiolene complexes. In some examples, a photoactive compound exhibitsa greater maximum near-infrared absorption strength than maximum visibleabsorption strength and has a chemical formula

In embodiments, each M is independently a metal, such as Fe, Co, Ni, Cu,Zn, Mo, Pd, or Pt. In embodiments, each R group is independently ahydrogen, a halogen, a cyano group, a substituted or unsubstituted alkylgroup, a substituted or unsubstituted alkenyl group, a substituted orunsubstituted aromatic group, a substituted or unsubstituted fusedaromatic group, a substituted or unsubstituted heteroaromatic group, asubstituted or unsubstituted fused heteroaromatic group, or asubstituted or unsubstituted alkoxy group. Optionally, two or more Rgroups form a substituted or unsubstituted ring group or a substitutedor unsubstituted fused ring group. In embodiments, each A isindependently a fused polycyclic ring group having one or more R groupsubstituents.

FIG. 7A and FIG. 7B provide example synthetic schema for making visiblytransparent photoactive compounds. With reference to FIG. 7A, it will beappreciated that many different substitutions may be made for thereactant

such as to allow substitution of the phenyl ring pendant to the dithionewith another structure structures, such as phenyl rings with more orsubstituents in different positions, or substitution of both phenylrings with other substituents, such as ring structures fused to thedithione

such that the resultant final product is different. Similarly, withreference to FIG. 7B, it will be appreciated that many differentsubstitutions may be made for the cyano groups in order to form adifferent final products. For example, substitutions may include phenylrings (e.g., alkyl substituted, halogen substituted, alkoxy substituted,etc.), fused ring structures. Other pendant ring structures may besimilarly added, such as naphthyl groups, phenanthryl groups,acenaphthyl groups, etc.

Table 1 provides example metal dithione complexes along with shorthandidentifiers (ID).

TABLE 1 ID Structure D-1

D-2

D-3

D-4

D-5

D-6

D-7

D-8

D-9

D-10

D-11

D-12

D-13

D-14

D-15

D-16

FIG. 8 provides exemplary normalized absorbance spectra for a number ofthe compounds in Table 1, showing that these compounds exhibit strongabsorbance in the near-infrared with some absorbance in the ultravioletand minimal absorbance in the visible band.

Sample photovoltaic devices were constructed using a number of thetransparent photoactive compounds in Table 1 and tested for photovoltaicefficiency. Two device configurations were used for testing purposes,generally constructed as depicted in FIG. 1A, with the transparentphotoactive compounds used as a photoactive layer or a buffer layer. Inone configuration, two transparent electrodes were used to generate afully transparent device. In a second configuration, one transparentelectrode (indium tin oxide) was used and one opaque electrode was used.Although devices constructed according to the second configurationexhibited opacity due to the presence of the opaque electrode, thedevice was illuminated through the transparent electrode to examine thephotovoltaic behavior.

As an example, a transparent photovoltaic device using the visiblytransparent photoactive compound identified as D-1 with an opaqueconfiguration was constructed as follows. An about 8 nm thick cathodebuffer layer was deposited over a visibly transparent substrate andindium tin oxide (ITO) transparent conductor. An about 15 nm thick layerof a subphthalocyanine (SubPc) dye as a donor was deposited over theanode buffer layer. An about 30 nm thick layer of compound D-1 as anacceptor was deposited over the donor layer. An about 5 nm thick cathodebuffer layer was deposited over the acceptor layer. Finally, an opaquealuminum layer about 80 nm thick was deposited as a top electrode. Thecell was illuminated and performance monitored. FIG. 9 provides exampledata for the D-10 photovoltaic cell, showing current density as afunction of voltage.

Another example transparent photovoltaic device was constructed using anickel dithiolate complex as a buffer layer in an opaque configuration.An about 10 nm thick cathode buffer layer including compound D-1 wasdeposited over a visibly transparent substrate and indium tin oxide(ITO) transparent conductor. As the photoactive layer, an about 60 nmthick layer including C₆₀ was deposited over the buffer layer. An about20 nm thick anode buffer layer was deposited over the acceptor layer.Finally, an opaque aluminum layer about 80 nm thick was deposited as atop electrode. The cell was illuminated and performance monitored. FIG.10 provides example data for the photovoltaic cell, showing currentdensity as a function of voltage.

Metal dithiolates can be used as acceptor materials and paired with anumber of useful donor materials. In some embodiments, for example, theacceptor material includes one or more metal dithiolates, and the donormaterial includes a boron-dipyrromethene (BODIPY) compound, aphthalocyanine, a naphthalocyanine, a dicyano-indandione (DiCN), abenzothiadiazole (BT), a benzo-bis-thiadiazole (BBT), adiketopyrrolopyrrole diphenythienylamine (DPP-DPTA), or a combinationthereof. Examples of useful BODIPY compounds include, but are notlimited to, those described in U.S. Provisional Pat. Appl. No.62/521,154 and the U.S. utility application filed on Jun. 15, 2018.Examples of useful DiCNs, BTs, BBTs, and DPP-DPTAs include, but are notlimited to, those described in U.S. Provisional Pat. Appl. No.62/521,224 and the U.S. utility application filed on Jun. 15, 2018.Examples of useful phthalocyanines and naphthalocyanines include, butare not limited to, those described in U.S. Pat. Appl. No. 62/521,214and the U.S. utility application filed on Jul. 15, 2018. Theseapplications are incorporated herein by reference in their entireties.In some embodiments, the donor material includes a BODIPY compound, aphthalocyanine, a naphthalocyanine, or a combination thereof. In someembodiments, the donor material includes a DiCN, a BT, a BBT, or aDPP-DPTA.

In certain instances, the acceptor material contains a metal dithiolate,and the donor material also contains a metal dithiolate. In suchinstances, the donor metal dithiolate may be functionalized with one ormore donor groups such as the strong, moderate, and weaker donatinggroups described above. Metal dithiolates functionalized in this mannercan also be used as donor materials when paired with an acceptormaterial such as a BODIPY, a phthalocyanine, a naphthalocyanine, a DiCN,a BT, a BBT, or DPP-DPTA, a naphthalenetetracarboxylic diimide (NTCDI),a bisimide coronene (CBI), a fluoranthene, a corannulene, a fullerene(e.g., C60 or C70), or a combination thereof. Examples of useful NTCDIs,CBIs, fluoranthenes, and corannulenes include, but are not limited to,those described in U.S. Provisional Pat. Appl. No. 62/521,211 and theU.S. utility application filed on Jun. 15, 2018. These applications areincorporated herein by reference in their entireties.

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

All references throughout this disclosure, for example patent documentsincluding issued or granted patents or equivalents; patent applicationpublications; and non-patent literature documents or other sourcematerial; are hereby incorporated by reference herein in theirentireties, as though individually incorporated by reference. Forexample, U.S. Pat. No. 6,120,692 and University of Rochester, Laboratoryfor Laser Energetics, LLE Review Quarterly Report, Volume 106,January-March 2006, Chapter 7: Transition Metal Dithiolene Near-IR Dyesand Their Applications in Liquid Crystal Devices, pp. 112-128, arehereby incorporated by reference.

All patents and publications mentioned in this disclosure are indicativeof the levels of skill of those skilled in the art to which theinvention pertains. References cited herein are incorporated byreference herein in their entirety to indicate the state of the art, insome cases as of their filing date, and it is intended that thisinformation can be employed herein, if needed, to exclude (for example,to disclaim) specific embodiments that are in the prior art. Forexample, when a compound is claimed, it should be understood thatcompounds known in the prior art, including certain compounds disclosedin the references disclosed herein (particularly in referenced patentdocuments), are not intended to be included in the claim.

When a group of substituents is disclosed herein, it is understood thatall individual members of those groups and all subgroups and classesthat can be formed using the substituents are disclosed separately. Whena Markush group or other grouping is used herein, all individual membersof the group and all combinations and subcombinations possible of thegroup are intended to be individually included in the disclosure. Asused herein, “and/or” means that one, all, or any combination of itemsin a list separated by “and/or” are included in the list; for example“1, 2 and/or 3” is equivalent to “‘1’ or ‘2’ or ‘3’ or ‘1 and 2’ or ‘1and 3’ or ‘2 and 3’ or ‘1, 2 and 3’”.

Every formulation or combination of components described or exemplifiedcan be used to practice the invention, unless otherwise stated. Specificnames of materials are intended to be exemplary, as it is known that oneof skill in the art can name the same material differently. It will beappreciated that methods, device elements, starting materials, andsynthetic methods other than those specifically exemplified can beemployed in the practice of the invention without resort to undueexperimentation. All art-known functional equivalents, of any suchmethods, device elements, starting materials, and synthetic methods areintended to be included in this invention. Whenever a range is given inthe specification, for example, a temperature range, a time range, or acomposition range, all intermediate ranges and subranges, as well as allindividual values included in the ranges given are intended to beincluded in the disclosure.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. Any recitation hereinof the term “comprising”, particularly in a description of components ofa composition or in a description of elements of a device, is understoodto encompass those compositions and methods consisting essentially ofand consisting of the recited components or elements. The inventionillustratively described herein suitably may be practiced in the absenceof any element or elements, limitation or limitations which is notspecifically disclosed herein.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments and optional features, modification and variation of theconcepts herein disclosed may be resorted to by those skilled in theart, and that such modifications and variations are considered withinthe scope of this invention as defined by the appended claims.

What is claimed is:
 1. A visibly transparent photovoltaic devicecomprising: a visibly transparent substrate; a first visibly transparentelectrode coupled to the visibly transparent substrate; a second visiblytransparent electrode above the first visibly transparent electrode; afirst visibly transparent photoactive layer between the first visiblytransparent electrode and the second visibly transparent electrode,wherein the first visibly transparent photoactive layer exhibits anaverage visible band absorption of 70% or less and comprises a firstphotoactive compound having the formula:

wherein the first photoactive compound exhibits a first maximumnear-infrared absorption strength and a first maximum visible absorptionstrength, wherein the first maximum near-infrared absorption strength isgreater than the first maximum visible absorption strength, and whereinabsorption of near-infrared light by the first photoactive compounddrives formation and separation of electron-hole pairs, M is a metal,and each R group is independently a cyano group, a substituted orunsubstituted alkyl group, a substituted or unsubstituted alkenyl group,a substituted or unsubstituted aromatic group, a substituted orunsubstituted fused aromatic group, a substituted or unsubstitutedheteroaromatic group, a substituted or unsubstituted fusedheteroaromatic group, or a substituted or unsubstituted alkoxy group, orwherein two or more R groups form a substituted or unsubstituted ringgroup or a substituted or unsubstituted fused ring group; and a secondvisibly transparent photoactive layer between the first visiblytransparent electrode and the second visibly transparent electrode andin direct contact with the first visibly transparent photoactive layer,wherein the second visibly transparent photoactive layer exhibits anaverage visible band absorption of 70% or less and comprises a secondphotoactive compound exhibiting a second maximum absorption strength inthe near-infrared or ultraviolet and a second maximum visible absorptionstrength, wherein the second maximum absorption strength is greater thanthe second maximum visible absorption strength, and wherein absorptionof near-infrared light or ultraviolet light by the second photoactivecompound drives formation and separation of electron-hole pairs, andwherein the first photoactive compound is an electron donor material foran organic photovoltaic cell and the second photoactive compound is anelectron acceptor material for the organic photovoltaic cell, or thefirst photoactive compound is an electron acceptor material for theorganic photovoltaic cell and the second photoactive compound is anelectron donor material for the organic photovoltaic cell.
 2. Thevisibly transparent photovoltaic device of claim 1, wherein each R groupis the same.
 3. The visibly transparent photovoltaic device of claim 1,wherein the second photoactive compound comprises aboron-dipyrromethene-based compound.
 4. The visibly transparentphotovoltaic device of claim 1, wherein the second photoactive compoundcomprises a phthalocyanine or a naphthalocyanine.
 5. The visiblytransparent photovoltaic device of claim 1, wherein the first visiblytransparent photoactive layer and the second visibly transparentphotoactive layer independently have thicknesses of 1 nm to 300 nm. 6.The visibly transparent photovoltaic device of claim 1, wherein thefirst visibly transparent photoactive layer and the second visiblytransparent photoactive layer correspond to separate, mixed, orpartially mixed layers.
 7. The visibly transparent photovoltaic deviceof claim 1, wherein the first photoactive compound is visiblytransparent.
 8. The visibly transparent photovoltaic device of claim 1,wherein the first visibly transparent photoactive layer is coupled tothe first visibly transparent electrode or wherein the first visiblytransparent photoactive layer is coupled to the second visiblytransparent electrode.
 9. The visibly transparent photovoltaic device ofclaim 1, wherein M is Fe, Co, Ni, Cu, Zn, Mo, Pd, or Pt.
 10. The visiblytransparent photovoltaic device of claim 1, wherein one or more R groupsare each independently a methoxy group, a cyano group, a thiazolylgroup, a phenyl group, a maleimide group, a naphthyl group, or aphenanthrene group.
 11. The visibly transparent photovoltaic device ofclaim 1, wherein absorption of near-infrared light by the firstphotoactive compound drives formation and separation of electron-holepairs and wherein absorption of near-infrared light or ultraviolet lightby the second photoactive compound drives formation and separation ofelectron-hole pairs.
 12. The visibly transparent photovoltaic device ofclaim 1, further comprising a visibly transparent buffer layer betweenthe second visibly transparent electrode and the first and secondvisibly transparent photoactive layers, wherein the visibly transparentbuffer layer comprises a sputtered inorganic buffer layer.
 13. Thevisibly transparent photovoltaic device of claim 1, further comprising avisibly transparent buffer layer between the second visibly transparentelectrode and the first and second visibly transparent photoactivelayers, wherein the visibly transparent buffer layer comprises asputtered metal oxide buffer layer.
 14. The visibly transparentphotovoltaic device of claim 1, wherein the second photoactive compoundexhibits the second maximum absorption strength in the near-infrared,and wherein first maximum near-infrared absorption strength occurs at alonger wavelength than the second maximum absorption strength in thenear-infrared.
 15. The visibly transparent photovoltaic device of claim1, wherein the second photoactive compound exhibits the second maximumabsorption strength in the near-infrared, and wherein the first maximumnear-infrared absorption strength occurs at a shorter wavelength thanthe second maximum absorption strength in the near-infrared.
 16. Thevisibly transparent photovoltaic device of claim 1, wherein the firstphotoactive compound is an electron donor material and the secondphotoactive compound is an electron acceptor material, wherein thesecond photoactive compound exhibits the second maximum absorptionstrength in the near-infrared and is selected from the group consistingof a molecule with an acceptor-donor-acceptor structure (A-D-A), amolecule with a donor-acceptor-donor structure (D-A-D), a QQT compound,a benzo-bis-thiadiazole compound, a dicyano-indandione compound, or ametal dithiolate compound different from the first photoactive compound.17. The visibly transparent photovoltaic device of claim 1, wherein thefirst photoactive compound is an electron donor material and the secondphotoactive compound is an electron acceptor material, wherein thesecond photoactive compound exhibits the second maximum absorptionstrength in the ultraviolet and is selected from the group consisting ofan indigo compound, a fluoranthene compound, a corranene compound, acorranulene compound, a naphthalenetetracarboxylic diimide compound, ora tetracyanoindacene compound.
 18. The visibly transparent photovoltaicdevice of claim 1, wherein the first photoactive compound is an electronacceptor material and the second photoactive compound is an electrondonor material, wherein the second photoactive compound exhibits thesecond maximum absorption strength in the near-infrared and is selectedfrom the group consisting of a squarine compound, a cyanine salt, amolecule with a donor-acceptor-donor structure (D-A-D), or a metaldithiolate compound different from the first photoactive compound. 19.The visibly transparent photovoltaic device of claim 1, wherein thefirst photoactive compound is an electron acceptor material and thesecond photoactive compound is an electron donor material, wherein thesecond photoactive compound exhibits the second maximum absorptionstrength in the ultraviolet and is selected from the group consisting ofa TAPC compound, an indigo compound, a fluoranthene compound, orcorranene compound.
 20. The visibly transparent photovoltaic device ofclaim 1, wherein each R is a halogen, a cyano group, or a substituted orunsubstituted alkenyl group.
 21. The visibly transparent photovoltaicdevice of claim 1, wherein each R is a substituted or unsubstitutedaromatic group, a substituted or unsubstituted fused aromatic group, asubstituted or unsubstituted heteroaromatic group, or a substituted orunsubstituted fused heteroaromatic group.